Impedance transformer



Nov. 26, 194e. A. G. Fox 2,411,534

IMPEDANCE TRANSFORMER Filed March' 3o, 1945 /NVENTOR A. G. FOX

Afro/wry Patented Nov. 26, i946 EMPEDANCE TRANSFORMER Arthur Gardner Fox, Morristown', N. J., assgnor to Bell Telephone Laboratories, incorporated, New York, N. Y., a corporation of New York Application March 30, i943, Serial No. 181,102

24 Claims. (Cl. 178-44) This invention relates to the transmission of guided electromagnetic waves and more particularly to an impedance transforming section for connecting two wave guides which dier in characteristic impedance.

The object oi the invention is to connect` together without reiiection two metal sheathed wave guides which have diierent characteristic imped-- construction which is suitable for this purpose.

The transformer comprises a section of metallic sheath having the same cross-section as that of the wave guides to be connected and a length approximately equal to a quarter wave-length. The section has a dielectric core having an eiective dielectric constant which is intermediate between thedielectric constants of the cores of the wave guides to be joined and of a value to provide an impedance match at each end of the section. The core ci the section may, for example, comprise a plurality of dielectric materials which diner in dielectric constants. One or more of these materials may be the same as used in the cores of the guides to be connected. The relative proportions of the materials are chosen to provide the desired impedance match. p

In one form oi the transformer one of the wave guides has a core of solid dielectric material and the other guide has a fluid core such, for example, as air or some other gas. The core in the interposed section may, for example, comprise one `or' more projections of the core of solid material. In some cases it is preferable that the 'projections y similar or corresponding parts and in which:

Fig. 1 is a perspective view, partly cut away, of

' one form of the impedance transforming section in accordance with the invention as applied to a sheath of rectangular cross-section;

Figs. 2 and 3 are longitudinal cross-sectional views of alternative forms of the transforming section shown in F18. l;

Fig. 4 is a perspective view, partly cut away, of another form oi the transformer;

Fig. 5 is a perspective view of a circular impedancey transforming section, cut in two longitudi nal f Fig. 6 is a longitudinal cross-section of still another iorm of the transformer; and

Fig. 7 is a longitudinal cross-section of a wave guide attenuator having at each end an impedance transformer o the type shown in Fig. l or Fig. 5.

Taking up the gures in more detail, Fig. l shows in perspective a wave guide comprising a metallic sheath l of uniform rectangular crosssection. The section 2 to the left has a core 3 of solid dielectric material such, for example, -as nitro-wax or polystyrene. The section i to the right has an air core. Since the cores of the sections 2 and 4 have different dielectric constants the `sections will differ in characteristic impedance. Therefore. in order to prevent wave reflection at the point of `iunction an impedance transforming section 5 must be interposed between the sections 2 and 4. In accordance with the invention, section 5 has a length A which is approximately equal to a quarter wave-length, for waves within the section and a dielectric core made up partly of the material of the core 3 of section 2 and partly of the core material of section 4, which is air. By properly choosing the relative proportions of the solid core material and air in section 5 a good impedance match may be obtained at each end of the interposed section 5.

- As shown in Fig. 1, the core 3 is extended into the section 5 in the form 'of two projections 6 and 'I which extend .from one side 8 to the opposite one 9 and are parallel with .and adjacent to the sides I0 and ll having the longer transverse dimension. The projections 6 and 1 may be conveniently formed by first filling section 5 with the material of the core 3 and then cutting out a rec-- tangular slot, as shown at I2. For some types of electromagnetic waves it is preferable that the slot I2 be centrally located, in order to preserve the symmetry of the structure.

The proper width BY for the slot I2' is perhaps best found by trial. A suggested procedure is to terminate the far end of section 2 in its characteristicimpedance and. to introduce radio frequencypower at the right of section 4. The standing waves set up in section 4 are then measured I `by a standing wave detector, and

tude and position noted. Starting with a rather narrow slot in section 5, this is widened in small steps. and at each step the standing wave is observed and the length of section 5 readjusted if necessary so that the interface between section 4 and section 5 will always be a position of voltage minimum in the standing wave pattern. Using this procedure a width of slot will finally be found xmore or less than two. may be used. Fig. 2, for

example, is a longitudinal cross-sectional view of an alternative form, similar to the one shown in Fig. 1 except that the two projections 6 and I are replaced by a single projection I3 which, for the sake of symmetry, is centrally located between the sides I and II. The projection I3 has a length A equal to a quarter wave-length and a width C chosen, as explained above, to provide an impedance match.

The transformer shown in the longitudinal cross-sectional view of Fig. 3 is similar to the one shown in Fig. 1 except that the projection I has been omitted. Although the structure is not symmetrical, it is suitable for use where only the dominant wave may be propagated in section 2 and section 4.

Fig. 4 is a perspective view of a transformer similar to the one shown in Fig. 1 except'that the two projections I4 and I5 are parallel with the sides 8 and 9, instead of the sides I0 and I I, and, instead of being adjacent to the sides with which they are parallel, are separated therefrom, Here again, any number of projections, either greater or less than two, may be used, and one or more may be adjacent to the side 8 or-9.

Fig. 5 is a'perspective view showing the invenmetallic sheath I5 of circular cross-section. 'Ihe impedance transforming section 5 is formed by drilling into the solid dielectric core 3 a coaxial hole Il of diameter D and depth A. Of course, any number of holes may be drilled. It is generally advisable, however, to locate the holes in such a way that axial symmetry is maintained.

Fig. 6 is a longitudinal cross-section showing how the invention may be applied to a wave guide comprising a sheath IB which may be either circular or rectangular in cross-section. The quarter Wave section 5 is filled with a solid dielectric material I9 in which a number of air pockets or bubbles 20 are formed. The air pockets 20 may, for example, be produced by some chemical process or by whipping a wax with a mechanical beater. Alternatively, the filler I9 may be an amorphous granular material with interstices 20 of air. Another method of obtaining the air pockets 20 is by mechanically removing portions of the solid material I9, as, for example, by drilling holes which may extend in any direction and go either part way or all the way through the material. In any case, a sufficient amount of air is introduced that the eective dielectric constant of the filler I9 is of the proper value to provide an impedance match at each end of section 5.

The longitudinal cross-sectional view .of Fig. 7 slzows the invention applied to a wave guide attenuator comprising a central section 2| to the ends of which are connected the impedance their amplitransforming sections 22 and 23 for joining the attenuator without refiection to air-filled wave guides. Each of the sections 2i, 22 and 23 has a metallic sheath 24 which may be either rectanguiar or circular in cross-section. The sections are joined together by means of the metallic bands 25 and 26 which preferably extend all the way around the sheaths 24. The central section v2| is filled with a solid dielectric material 21.

Each of the end sections 22 and 23, for a distance A equal to a quarter wave-length, is filled partlyv with the solid dielectric material 2l and partly with air. If the sheath 24 is of rectangular cross-section, the transformer sections 22 and 23 may be of the form shown in Fig. 1, 2, 3 or 4, or some modification thereof. If the sheath 24 is circular, the end sections 22 and 23 may. for example, be of the form shown in Fig. 5. Also, of course, with either a rectangular or circular sheath 24 the sections 22 and 23 may be of the type shown in Fig. 6. vThe length E of the central section 2| is chosen to give the desired overall attenuation for the attenuator.

What is claimed is:

1. An impedance transforming section for connecting together without refiection two wave guides, said section and each of said guides cornprising a metallic sheath of the same cross-section, one of said guides having a core of solid dielectric material, the other of said guides having a core of dielectric material which has a dielectric constant different from that of said core in said one guide, and said section having a length approximately equal to a quarter wave-length and a dielectric core which has an effective dielectric constant of a value to provide an impedance match at eachv end of said section.

2. An impedance transforming section in accordance with claim 1 in which said core of said -section comprises a plurality of dielectric materials which differ in dielectric constants,

3. An impedance transforming section in accordance with claim 1 in which said core of said section comprises a solid dielectric material and a fluid dielectric material.

4. An impedance transforming section in accordance with claim 1 in which said core of said 6. An impedance transforming section in accordance with claim 1 in which said core of said section is made partly of the material of said core in said one guide and partly of the material of said core in said other guide.

7. An impedance trans/forming section in accordance with claim 1 in which said core in said other guide is a gas.

8. An impedance transforming section in accordance with claim 1 in which said core in said other guide is a gas and said core of said section is made partly of solid material and partly of gas.

9. An impedance transforming section in accordance with claim 1 in which said core in said other guide is air and said core of said section is made partly of the material of said core in said one guide and partly of air.- Y

10. In a metal sheathed wave guide of uniform cross-section, a first section having a core made of a, solid dielectric material, a second section having a core of dielectric material which has a comprises a plurality of dielectric materials which differ in dielectric constants.

12. The combination in accordance with claim 10 in which said core of said interposed sectioncomprises a solid dielectric material and air.

13. The combination in accordance with claim 10 in which said core of said interposed section is made partly of the material of said core in said first section and partly of the material of said core in said second section.

14. The combination in accordance with claim loin which said core in said second section is air and said core of/said interposed section is made partly of the material of said core in said first section and partly of air.

15. The combination in accordance with claim 10 in which said core in said interposed section comprises a projection of said core in said first section. f

16. The combination in accordance with claim l in which said core in said interposed section comprises a symmetrical projection of said core in said rst section.

17. The combination in accordance with claim in whichsaid core in said interposed section comprises a plurality of projections of said core in said first section.

18; The combination in accordance with claim 10 in which said core in said interposed section comprises a plurality of symmetrical projections of said core in said first section. l 19. The combination in accordance with claim 10 in which said core in said second section is air and said core in said interposed section comprises a projection of said core in said first section.

20. The.combination in accordance with claim 10 in which said core in said second section is air and said core in said' interposed section comprises a plurality of proiectionsof said core in said firstsection.

21. A metal sheathed wave guide of uniform cross-section containing a solid dielectric ele`- ment and a medium having a different dielectric constant, saidlelement having a quarter wavelength terminal portion of reduced cross-section extending into said medium for matching the impedance of the section of the guide containing said element to said medium. 22. A wave guide in accordance with claim 21 in which said terminal portion is of uniform cross-section.

23. A metal sheathed'wave guide of uniform cross-section containing a solid dielectric element and a medium having a different dielectric constant. said element having a quarter wavelength terminal portion of reduced cross-section extending into said medium and the section of the guide containing said terminal portion having an effective dielectric constant of a value to provide an impedance match at each end thereof.

24. A metal sheathed wave guide of uniform cross-section containing a solid dielectric ele; mentand a medium having a dierent dielectric constant, said element having at one end a longitudinal slot extending to a depth of a quarter wave-length and the medium extending into said slot together with the material in which said slot is formed combining to provide an effective 'dielectric constant of proper value to match the impedance oi' the section of the guide containing said element to said medium.

ARTHUR GARDNER FOX. 

